1. Field of the Invention
The present invention relates to improvements in the stop-and-shutter device of a camera and, more particularly, to an electromagnetically driven shutter device which is powered by an electromagnetic force obtained by fixed permanent magnets and moving coil members to move one or more shutter blades for opening or closing the stop.
2. Description ot the Prior Art
Generally, electromagnetically driven shutter devices can fully control the velocity, direction, and so on of driven shutter blades by the direction and amount of the electric current fed to a moving coil member. Therefore, they do not require a complicated mechanism, such as a mechanical shutter driven by a spring force, and dispense with a delay governor mechanism consisting of a combination of gears and a sector wheel. Further, they are thin and compact.
In known electromagnetically driven shutter devices, however, shutter blades are driven only one moving coil member interposed between permanent magnets and a yoke to make the gap between the magnets and the yoke as narrow as possible for reducing the loss in the magnetic circuit. Accordingly, it is possible for a program shutter having stop-and- shutter blades serving as a stop , or a diaphragm, and also as the tripping means of the shutter to move from the closed side to the open side and the vice versa in a reciprocating manner. However, it is impossible for such a shutter to move the blades for opening or closing the shutter while keeping the diameter ot the aperture of the stop at a specific value, unless a separate and complicated delay governor mechanism is provided. Thus, conventional electromagnetically driven shutter devices cannot help but adopt programs in which the diameter of the aperture ot the stop and the time required for the shutter to complete its motion are related to each other such that they draw triangles when the diameter is taken on the ordinate and the time is taken on the abscissa of a graph. For this reason, it has been heretofore difficult to take a photograph taking into account the effect of a stop such as encountered when the focal depth is optimized, unless the diameter happens to coincide with the value set by a uniquely determined program. In this way, the conditions under which photographs can be taken are quite limited.
Also in the conventional, electromagnetically driven shutter devices having the fixed permanent magnets, the yoke disposed opposite to the magnets, and the movable coil member interposed between these components, the movable coil member is driven by the electromagnetic force which is produced by energizing it. When the coil member is not energized, i.e., when no electromagnetic force is produced, it is free to move. Hence, it is customary to use a spring to accurately hold the coil member in its initial position. It is therefore required that the coil member be driven against the spring force.
Unfortunately, as the coil member is moved a greater distance, a greater force must be exerted by the spring. This requires that a correspondingly larger electric current be supplied to the coil member, thereby rendering the associated control circuit more complicated. This problem may be solved by feeding a large current from the first, but it is not preferable to use such a large current, because this greately shortens the life of a small battery generally used in a camera.